This invention relates to a so-called electronic distribution type ignition system which has done away with the distributor for supplying a high voltage from the ignition coil to the spark plugs, and more particularly to an electronic distribution type ignition system suitable for multicylinder engines such as automotive gasoline engines.
In engines incorporating a plurality of cylinders, such as automotive gasoline engines, for example, it has been a general practice to use a distributor for high-voltage power supply from the ignition coil to the spark plugs. Lately, an electronic distribution type ignition system has been used commercially which has one-to-one or two-to-one combinations of spark plugs and ignition coils by which ignition is controlled without using a distributor.
In the electronic distribution type ignition system, it is necessary to provide each ignition coil with a power transistor for control of power supply to the ignition coil, and a current-detecting resistance element and a feedback control circuit to control the primary current flowing through the ignition coils at a predetermined value.
In designing an ignition system to comply with the above requirement, it has been practiced to put the control circuit and circuit elements into common use for a plurality of ignition coils in order to realize a whole ignition control circuit in a reduced size and decrease production cost.
One example of this can be seen in JP-A-60-09667 for which the patent application has been filed on Apr. 2, 1984 and laid open later.
According to the disclosure of this publication, it has been proposed to provide a common current detection circuit for a plurality of power transistors for power supply control of the ignition coils and for feedback control circuits.
The circuit configuration disclosed in this publication has one current detection circuit, and at a glance, the circuit looks as if it has been simplified so that the circuit area can be reduced. However, the use of one current detecting circuit requires longer wires to provide detection signals from the current detection circuit to the four feedback control circuits. Stretching wires around increases wire intersections and closeness of wires, which increase the occurrence of electromagnetic interference and noise. Furthermore, in such a circuit configuration, it is necessary to use additional elements such as diodes to prevent interference among the circuits. In consequence, the common use of a current detection circuit rather increases the whole area of the circuit and production cost, giving rise to an increase in noise on top of that. This problem becomes more serious as the number of cylinders increases.